Process and arrangement for selective network monitoring for switchgear

ABSTRACT

In switchgear with superordinate power circuit-breakers and a plurality of subordinate power circuit-breakers, the superordinate power circuit-breaker must be triggered in the event of failure of the subordinate power circuit-breakers. This is accomplished as follows: electrical post-arc currents are detected after current zero, the electrical post-arc currents are compared with a predetermined limit value, and if the limit value is exceeded, a signal is effected for triggering the superordinate power circuit-breaker in the switchgear. This method can advantageously be implemented in particular in the case of vacuum circuit-breakers with at least one vacuum interrupter. The associated arrangement has a device which monitors the subordinate power circuit-breakers and, if appropriate, triggers the superordinate power circuit-breaker.

FIELD OF THE INVENTION

The present invention relates to a method for selective networkmonitoring for switchgear. In addition, the present invention alsorelates to the associated arrangement for implementing the method.

BACKGROUND INFORMATION

In power distribution installations, one of the most importantdisruptions occurring in practice is a short circuit in subdistributionnetworks which leads to short-circuit currents which are generally morethan one order of magnitude above the rated currents in the network. Notonly can such short circuits lead to local damage due to the arc that isgenerally produced in the event of a short circuit, but also they impairthe security and functioning of the superordinate network. For thisreason, power circuit-breakers are used in subdistribution networks, inorder, in the event of disruptions in the network, to isolate theaffected part of the network, i.e. the subdistribution board with loadsin this respect, from the remaining region and, in this way, ensureunimpaired functioning of the overall system. These powercircuit-breakers have the task of interrupting the short-circuit currentafter only a few current half-cycles, in order to minimize any damageand impairment of the network that occur. The power circuit-breakersgenerally comprise a stationary contact and a contact that can be movedrelative thereto, which open a previously closed electric circuit bymechanically separating the contact elements.

In exceptional cases, failure of the power circuit-breaker may occurwhen, by way of example, the switching contacts have reached the end oftheir service life. In this case, a superordinate power circuit-breakermust isolate a correspondingly larger part of the subordinate networkfrom the rest of the network. Problems that are posed here involveidentifying the failure of the subordinate power circuit-breaker with ahigh degree of reliability and very early on, in order that thesuperordinate power circuit-breaker is triggered with a sufficientlyshort time delay. Especially in the case of vacuum circuit-breakers,when so-called late failures occur, i.e., events in which the powercircuit-breaker initially separates successfully but then fails due toan arc restriking, diagnosis of the switching behavior is very importantin order to lead with certainty to the triggering of the superordinatepower circuit-breaker.

In practice, conventional methods which identify a short circuit onaccount of the current amplitude in the network and, if appropriate,generate a corresponding signal for triggering the superordinate powercircuit-breaker. According to this method, it is generally necessary toeffect measurement over a number of current half-cycle durations inorder to obtain a sufficiently high signal-to-noise ratio in particularwith respect to false triggering. This method has the disadvantage that—in particular in the event of failure of the subordinatecircuit-breakers— the short-circuit current has to flow for a number ofhalf-cycles in order to be able to be identified with certainty. Thismeasure is necessary in order to reliably distinguish operationalovercurrents from short circuits. This means, however, that damage mayalready be produced at the location of the short circuit and thedisturbance may propagate widely through the power distribution network.

Furthermore, early identification of a short circuit by simultaneousanalysis of the rate of current rise in addition to the currentamplitude is described in U.S. Pat. No. 4,811,154. A method for earlyidentification of a short circuit by means of digital algorithms is alsodescribed in etz, Vol. 112 (1991), pp. 718-722. Early identification ofa short circuit is thus possible as early as in the rising part of ahalf-cycle with the instantaneous current still being comparativelysmall. However, in this case, too, the failure of a powercircuit-breaker can be detected only in combination with the detectionof an inadequately long arc duration in the relevant circuit-breaker.Especially in the case of multipole switching devices, it is thereforenecessary to concomitantly measure the arc duration in all poles.Corresponding diagnosis as well as the early identification of a shortcircuit itself are therefore very complicated and cost-intensive. Inparticular, late failures cannot be identified until they occur and, forthis reason, at the very least cannot be predicted.

SUMMARY

An object of the present invention, therefore, is to provide a methodand an associated arrangement which enables improved selectivity in thenetwork monitoring for switchgear.

This object is achieved by providing the following features:

electrical post-arc currents are detected after current zero,

the electrical post-arc currents are compared with a predetermined limitvalue, and

if the limit value is exceeded, a signal is generated for triggering asuperordinate power circuit-breaker in the switchgear.

In the associated arrangement in which the network of a switchgear hasat least one superordinate power circuit-breaker and a plurality ofsubordinate power circuit-breakers, a device is present which monitorsthe subordinate power circuit-breakers and, if appropriate, triggers thesuperordinate power circuit-breaker. For this purpose, the devicecomprises units for state identification and current measurementincluding the detection of the post-arc current of the powercircuit-breakers, on the one hand, and a release for the superordinatepower circuit-breaker.

The solution according to the present invention is based, therefore, onthe measurement of the post-arc currents using structural elements ofthe power circuit-breakers. This is possible, in particularly, using thevapor shields in vacuum circuit-breakers. The level of the post-arccurrents is a measure of the dielectric recovery of an interrupterfollowing successful disconnection of a short-circuit current at the endof a current half-cycle. Using an extensive series of experiments, ithas been possible to demonstrate that, above an experimentallyconfirmed, sharply defined limit of the post-arc current after thecurrent zero crossing, even if disconnection is initially successful,failure of the relevant interrupter must be expected with a highprobability. On the other hand, a reliable isolating behavior is ensuredbelow this limit. The absolute position of this limit is dependent onthe design but, specifically in the case of vacuum interrupters, lies ina range of from about 5 to 15 A which is readily accessible tomeasurements.

If the present invention is applied to vacuum interrupters, inparticular, a prediction regarding the time response in the case ofinterrupters with a shield that is linked on one side, i.e., with anelectrical connection between the vapor shield and one of the twoelectrodes, is also made possible, even if the interrupter does not failuntil after a few 100 ms due to a late failure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a graphical plotting of the maximum post-arc currentspecifically during the opening of vacuum circuit-breakers, plottedagainst the arc duration.

FIG. 2 shows a schematic illustration of a vacuum circuit-breaker inwhich the measurements according to FIG. 1 were carried out.

FIG. 3 shows an arrangement for selective network monitoring for aspecifically single-pole power circuit-breaker.

FIG. 4 shows a modification of the arrangement according to FIG. 3 witha bus system.

A method is provided in which, specifically in the case of vacuumcircuit-breakers in detail, the current is measured and analyzed afterthe first current zero crossing of the opening switching pole, in orderto generate a signal for triggering the superordinate powercircuit-breaker in the event of a preset threshold value being exceeded.Consequently, the sub-region of the network with the failing powercircuit-breaker can be isolated from the network very early on— as amatter of fact even before the actual failure under certaincircumstances in the case of late failures. As a result, a degree ofsecurity in power distribution networks is achieved which was notpossible using previous methods. In particular, instances of damage andhazards due to the arc occurring in the region of the short circuit aregreatly reduced and consequential costs and emissions, such as soundand/or arc products, are minimized.

FIG. 1 reproduces the results of extensive investigations. FIG. 1 showsa plot experimentally measured post-arc currents which relate torestriking or late failures as a function of the arc duration, thisbeing identified by filled-in symbols, and the post-arc currents whichwere measured in the case of entirely satisfactory opening withoutrestriking, this being identified by symbols that are not filled in. Theorder of magnitude of the post-arc currents lies in the range of fromabout 5 to 15 A, and the measurement times in the ms range. It isevident from the illustration according to FIG. 1 that there is a limitbelow which a reliable isolating behavior is ensured, whereas above thelimit the failure of the relevant interrupter must be expected with highprobability even if disconnection is initially successful.

The measurements for the illustration according to FIG. 1 were performedspecifically on a vacuum interrupter, reproduced schematically in FIG.2. In this case, 1 denotes the stationary power supply bolt and 2 themovable power supply bolt, the bolts carrying at their ends a stationarycontact 11, on the one hand, and a movable contact 21, on the otherhand. The contacts are arranged inside the interrupter 30, which ishermetically closed off with respect to the outside and comprises avapor shield 31, which forms the encapsulation as it were and isconnected to the stationary power supply bolt 1, an insulator 32 andbellows 33. Such interrupters are known with all sorts of variations inpractice.

FIG. 3 illustrates a line network in which a series of subdistributionboards or loads which can each be switched in or out by means of adedicated, subordinate power circuit-breaker are protected by asuperordinate power circuit-breaker. The superordinate powercircuit-breaker is designated by 2, while the two subordinate powercircuit-breakers illustrated in FIG. 3 have the reference symbols 21 and22 and the loads connected downstream have the reference symbols 31 and32.

In the event of a short circuit in one of the subdistribution boards,the associated subordinate power circuit-breaker 21 or 22 must open inorder to avoid a disturbance in the other subdistribution networks. Ifthe corresponding power circuit-breaker 21 or 22 fails, then thesuperordinate power circuit-breaker 2 must isolate the total quantity ofsubdistribution boards assigned to it from the rest of the network. Itreceives the triggering signal necessary for this purpose from a device5 for selective network monitoring, which is described in detail below.

According to FIG. 3, the device 5 for selective network monitoringincludes two parts: a diagnostic part 6, comprising a unit 7 foridentifying the state of a circuit-breaker and a unit 8 for currentmeasurement, and also a release 9 with corresponding intelligence forevaluating the information acquired from the diagnostic part 6.

In FIG. 3, the state identification unit has the task of identifying thecase where switching contacts have opened. In the case of contacts thathave opened, a signal is emitted to the triggering circuit. The currentmeasurement in this case specifically pursues two aims: on the one hand,successful disconnection must be diagnosed, which can be effected byidentifying a current zero crossing in the simplest case. On the otherhand, is necessary to identify whether the post-arc current flowingafter disconnection in the corresponding phase lies above the criticalcurrent limit for the interrupter, beyond which limit a failure must beexpected with high probability. If both preconditions are met, that isto say disconnection took place and an excessively high post-arc currentoccurred, the release 9 receives a corresponding signal.

The triggering circuit outputs a trigger signal to the superordinatepower circuit-breaker precisely when the two input signals of stateidentification and current measurement are present at the same time. Asa result, the subdistribution boards supplied by the superordinate powercircuit-breaker are isolated from the network, so that the path with thefailing power circuit-breaker is de-energized and the short circuit inthe region of the subdistribution board is not supplied with any furtherenergy despite a failing power circuit-breaker.

The selective network monitoring is illustrated only once as a commonunit 5 in FIG. 3. In actual fact, dedicated monitoring must be presentin each of the subordinate power circuit-breakers 21 and 22 involved. Inthis case, it may be expedient in accordance with FIG. 3 for only thetwo functions “state identification” and “current measurement” to beseparately assigned to each circuit-breaker 21, 22 and for a singletriggering unit to be used, which is assigned to the associatedsuperordinate power circuit-breaker 2. A corresponding bus system may bepresent for the purpose of communication.

In FIG. 4, the arrangement of FIG. 3 is modified in such a way thatunits 61, 62 and 63 for current measurements, state identification andtriggering, which are coupled via a bus system 10 to a unit 51 for theselective network monitoring, are respectively assigned to the systemcomprising superordinate power circuit-breaker 2 and subordinate powercircuit-breakers 21, 22 with loads 23, 24 respectively connecteddownstream. Comprehensive communication is thus ensured.

The method described and the associated arrangement have been explainedabove specifically for a vacuum circuit-breaker with a singleinterrupter, that is to say for a single-phase network. In the case ofthree-phase networks, a separate current measurement is carried out foreach phase of the subordinate power circuit-breaker.

Apart from vacuum circuit-breakers, the method can also be applied toother switching principles, provided that a comparable sharply definedlimit corresponding to an imminent failure likewise exists for thecurrent in those cases.

We claim:
 1. A method for selective network monitoring for switchgear,comprising the steps of: detecting electrical post-arc currents aftercurrent zero; comparing the electrical post-arc currents to apredetermined limit value; and if the electrical post-arc currentsexceed the predetermined limit value, generating a signal triggering asuperordinate power circuit breaker in the switchgear.
 2. The methodaccording to claim 1, wherein the detecting step includes the step of:measuring and analyzing the electrical post-arc currents after thecurrent zero, the current zero being a first current zero crossing of anopening switching pole.
 3. The method according to claim 1, wherein thepredetermined limit value depends on a design of the switchgear.
 4. Themethod according to claim 1, wherein the switchgear includes vacuumcircuit-breakers with at least one vacuum interrupter, and wherein thedetecting step includes the step of: measuring the post-arc currentsusing a structural element of the vacuum interrupter.
 5. The methodaccording to claim 4, wherein the structural element includes a vaporshield.
 6. The method according to claim 4, wherein the predeterminedlimit value is between 5 A and 15 A.
 7. An arrangement for selectivenetwork monitoring for switchgear, comprising: a superordinate powercircuit breaker; a plurality of subordinate power circuit breakers; amonitoring device monitoring the subordinate power circuit breakers andtriggering the superordinate power circuit breaker if electricalpost-arc currents detected after current zero exceed a predeterminedlimit value.
 8. The arrangement according to claim 7, furthercomprising: dignostic units assigned to the monitoring device, thediagnostic units for state identification and current measurement of thesuperordinate power circuit breaker and the subordinate power circuitbreakers.
 9. The arrangement according to claim 7, further comprising: arelease coupled to the superordinate power circuit-breaker.
 10. Thearrangement according to claim 7, further comprising: dedicateddiagnostic units, each of the subordinate power circuit breakers beingassigned a respective one of the dedicated diagnostic units, thediagnotic units for state identification and for current measurement;and a triggering unit for triggering the superordinate powercircuit-breaker.
 11. The arrangement according to claim 9, furthercomprising: a bus system for data transmission coupled to at least oneof the superordinate power circuit breaker, the subordinate powercircuit breakers, and the monitoring device.